Intra-annual dynamic of NSCs
The values and the modelled intra-annual dynamics of NSCs (total sugars,
starch and soluble sugars content) measured in the beech stemwood are
reported in Fig. 3 (panels a, b and c), while the intra-annual
statistical differences and the multiple comparisons for 2016 and 2017
are shown in Fig. S4.
Dynamic of NSCs showed polynomial equation patterns at different grades,
with R2 ranging from 0.64 to 0.93 (Table 1). Comparing
the modelled NSCs intra-annual dynamics and stand phenology, an increase
in total NSCs is observed from the bud break to the beginning of
green-up phase, due to the increasing starch content notwithstanding the
decrease of soluble sugars. During the period between the onset and the
middle of the maximum vegetative season, total NSCs content decreased
due to starch reduction, while the amount of soluble sugars remained
unchanged. In the late summer, both starch and soluble sugars increased
until the end of the vegetative season, determining an increase of total
sugars content (Fig. 3a). At the beginning of dormancy phase, a decrease
of total NSCs was recorded, driven by a severe decrease of starch
although associated with a simultaneous increase in soluble sugars.
In 2016, during the leafless period soluble sugars content significantly
decreased from 8.15 ± 0.58 mg g DW–1to the lowest value (5.25 ± 0.43 mg gDW–1), which was measured close to
the build-up of the new photosynthetic apparatus. Before the senescence,
a peak of soluble sugars content was observed (241 vs 215 Day of the
Year, DOY), while during the dormancy phase the maximum soluble sugars
content (16.75 ± 0.50 mg g DW–1) was
assessed. A significant increment of starch content was observed from
the beginning of the vegetative period (13.35 ± 1.89 mg gDW–1) until after the canopy
destruction (24.80 ± 0.20 mg g DW–1).
Subsequently, a decline in the starch content was observed before a
second seasonal peak (DOY 241), while the lowest starch content was
recorded during the dormancy phase (9.37 ± 0.78 mg gDW–1). In 2016, two seasonal peaks of
stemwood total sugars content were observed, with a significant increase
(110 vs 151 DOY) and decrease (151 vs 215 DOY) in spring and summer,
respectively. At the end of the vegetative season (241 vs 215 DOY) a
significant increment of total sugars was recorded.
In 2017, during spring and early summer a significant decrement of
stemwood content of soluble sugars until the lowest seasonal value (6.16
± 0.41 mg g DW–1, DOY 186) was
measured. A significant increment of soluble sugars content was observed
between July and August (218 vs 241 DOY), before reaching the maximum
value of 15.27 ± 0.76 mg g DW–1during the senescence. In spring and early summer 2017, stemwood starch
content significantly increased from 11.57 ± 1.51 mg gDW–1 to 20.26 ± 1.92 mg gDW–1. At the end of August, a
significant decrement of starch content was observed (12.76 ± 1.92 mg gDW–1, DOY 241). Stemwood total sugars
were lower at the beginning of the vegetative period (23.51 ± 2.38 mg gDW–1) than during the senescence,
when the maximum value was reached (31.94 ± 3.03 mg gDW–1).
Results from the linear mixed model showed that sampling dates account
for 52, 43, and 80 % of the total variance of starch, total NSCs and
soluble sugars, respectively. Meanwhile, trees explain 8, 11, 1% of the
total variance of concentration of starch, total NSCs and soluble
sugars, respectively.
Although the seasonal trends of stemwood carbohydrates accumulation in
2016 and 2017, were similar to the modelled NSCs dynamic recorded in the
reference period, some substantial differences can be observed. After
the leaf re-sprouting in 2016, starch content decreased considerably,
reaching a value lower than the modelled reference value at the site. In
August 2016, soluble sugars increased until leaves senescence. After
that, a reduction of starch and consequently of the total carbohydrate
reserves was observed. The lower amount of storage carbohydrates reached
in 2016, directly affected the starch amount recorded during the first
part of the vegetative season in 2017. At the beginning of August 2017,
although the starch content was lower than the modelled value of the
site, a refilling of total carbohydrate reserves was observed. The
drought stress event of August 2017 strongly affected the composition of
carbohydrate reserves due to a severe starch hydrolysis, leading to a
decrease of starch content of about 35% and a parallel increase of
soluble sugars. During the late phase of the 2017 vegetative season, the
carbohydrates pattern returned close to the modelled intra annual
dynamic, although during the dormancy stemwood starch content was lower
than the modelled value of the site.